Synthetic reverberation systems



Nov. 15, 1966 R. H. DORF I SYNTHETIC REVERBERATION SYSTEMS Filed June18, 1962 A 5 Sheets-Sheet 1 PLAYBACK PLAYBACK PLAYBACK ERASE RECORD NoN02 N03 5 42 25 3'6 512 4b i l 2 f; 5 BIAS ERASE OSCILLATOR 4 RECORD PBPB PB AMP. AMP. AMP. AMR g No I No 2 No 3 5 FEEDBACK ISOLATION CONTROL 5AMP. AMP AME AMP.

ISOLATION OUTPUT AMP. AMP. I6 5 RICHARD H. DORF FIG.|

Nov. 15, 1966 R. H. DORF 3,286,042

SYNTHETIC REVERBERATION SYSTEMS Filed June 18, 1962 5 Sheets-Sheet 5 Aon RICHARD H. DORF HPHI #3 zwmm W Zn JmaDm KmBOm 9m: F l cmooum IT ATTRNEY United States Patent C 3,286,042 SYNTHETIC REVERBERATION SYSTEMSRichard H. Dorf, New York, N.Y., assignor to Schober Organ Corporation,a corporation of New York Filed June 18, 1962, Ser. No. 203,252 5Claims. (Cl. 179-100.2)

This invention relates to synthetic reverberation systems andparticularly to improvements in electrical reverberation systems of thetype having record means and selectively delayed playback means forcontinuously recording and playing back a signal representative of soundenergy to generate reverberation components, which are combined with anunrecorded portion of said signal to form a composite reverberatedsignal.

Heretofore, record-playback reverberation systems have generallycomprised means for recording a signal to be reverberated on an endlessrecord material such as magnetic tape. A plurality of playback devicesare disposed about the path of travel of the recording medium in amanner such that the output signal component from each playback devicehave selected time spacings and amplitudes to form corresponding pointson a selected reverberation time curve for the system. A portion of thesignal output from one of the playback devices is usually fed back tothe recording device so that it may be re-recorded and the whole cyclerepeated. The system thus provides a number of repetitions of theoriginal sound which successively decrease in magnitude to conform tothe desired reverberation curve.

It is customary in synthetic reverberation systems of this type toinitially align and adjust the system for a single, predeterminedreverberation time. Since the output signal from the record-playbackdevice is combined with a direct signal, control of reverberation timeis then effected by adjusting the magnitude of the signal which passesthrough the reverberation channel with respect to that which passesthrough the direct channel. When other than the predeterminedreverberation times is desired, the effect of controlling reverberationin this manner has been generally unsatisfactory a the effect created isunnatural and artificial.

For such predetermined reverberation time adjustments, each successiverepetition decreases in amplitude a substantially uniform amount andthus usually conforms to the reverberation curve which closely simulatesa desirable natural curve produced acoustically, since the originalsound ceases at its maximum amplitude and fades away at a uniform rate.However, when control of reverberation time is attempted by merelyreducing the signal level at the input or output of the channel whichproduces the repetitions, an abrupt change in sound level ofconsiderable magnitude is heard by a listener to the system aftercessation of the direct signal. Furthermore, the slope of the decaycurve, though lower in absolute level, remains the same as at theinitially selected reverberation time, since the differences inmagnitude between the repetitive signal components have not been changedby such a type of control. No parallel in nature can be found for thiscondition and the ear merely interprets it as the result of an abruptcessation of sound followed by an echo.

In order to overcome this problem, it has been proposed to control thereverberation for time periods less than that initially predetermined asdescribed above, by adjusting the amplitude of the feedback signal sothat the second and all successive sets of repetitive signals initiate-dby the feedback control are commenced at an amplitude set selectively bythe magnitude of the feedback signal. ment over merely reducing the gainof the reverberation Although this type of control is an improve-3,286,042 Patented Nov. 15, 1966 channel, yet it is still artificial inquality since the abrupt changes in magnitude between each set ofrepetitive signals initiated by the recording of the feedback signalproduce sudden changes in signal level which are heard by the ear as aseries of echoes with results similar to those of the system previouslymentioned.

It is therefore a primary requirement for a realistic syntheticreverberation system, if it is to have means for varying thereverberation time and yet simulate the effect occurring in the physicalcreation of sounds in rooms having different reverberation times, thatthe continuation from each signal repetition to the next besubstantially uniform for all reverberation times. Thus, the decay curvemust remain smooth, but its slope must change with shorter reverberationtimes, so that the signal decrement rate is constant commencing at theinstant when the original, direct signal ceases.

It is therefore an object of the invention to provide a syntheticreverberation system in which the slope of the reverberation decay curveis changed in response to selected changes in the reverberation time ofthe system.

It is another object to provide a synthetic reverberation system havinga variable reverberation signal decay time in which the difference inmagnitude between successive repetitive signals is substantiallyconstant for each selected decay time.

It is still another object of the invention to provide means forsmoothly varying the reverberation time in a synthetic reverberationsystem by changing the slope of the reverberation decay curve withoutintroducing abrupt changes in magnitude into the reverberated signaloutput.

In accordance with the broader aspects of the present invention, meansare provided for recording on an endless record material such asmagnetic tape, a signal to be reverberated. A direct signal channel isalso provided. A plurality of playback heads are positioned along thepath of travel of the recording medium in a manner such that the outputsignal components from each playback head have a selected time spacingand amplitude. The signal from the last playback head is fed back to therecording device at a selected amplitude. Means are further provided foradjusting, with respect to the direct channel, the respective amplitudesof the signals from each of the playback heads, and from the feedbackchannel, such that the difference in amplitude between successivesignals is uniform. A reverberation time control for varying thereverberation time of the system is provided which varies the magnitudeof each reverberation signal component accordingly while maintaining auniform difference magnitudes of successive time spaced signals. Thus,for each change in the setting of the reverberation time control, themagnitudes of all reverberation signal components are changed an equalamount in order to maintain uniform the attenuation between successivesignals.

An additional feature of the present invention is the provision of anovel input and output gain control arrangement for the system whichmaintains constant the gain and signal-tO-noise ratio of the system overwide range of signal input magnitude variations.

For a better understanding of the invention, together with other andfurther objects thereof, reference is made to the following detaileddescription taken in connection with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of a synthetic reverberation systemconstructed according to the invention.

FIGS. 2a, 2b are graphs illustrating reverberation characteristicsobtainable with prior art apparatus.

FIG. 3 is a graph illustrating reverberation characteristics obtainablewith the system of FIG. 1.

FIG. 4 is a schematic diagram of a synthetic reverberation systemconstructed according to the present invention.

Referring now to the drawings, there is shown in FIG. 1 in blockschematic form, a reverberation system in accordance with the presentinvention which has a direct channel and a reverberation channel 12. Thedirect channel 10 has a pair of input terminals 11 to which may beconnected any suitable source of sound to be reverberated such as aradio receiver, or an electronic musical instrument. Channel 10 also hasa pair of output terminals 16 which may be connected to other amplifiersor incorporated into an audio system or may be connected directly toloudspeakers, if desired. Terminals 11 are connected to the input of anisolation amplifier 18, the output of which is connected to aconventional amplifier 20 which may be a cathode follower amplifier toprovide a low impedance output, if desired. As the signal applied toinput terminals 11 passes through direct channel 10 without alteration,then, if the remainder of the system becomes inoperative, the originalsignal will be heard without change.

Also connected to the input of isolation amplifier 18 is the input of asecond isolation amplifier 22 which may be a conventional voltageamplifier and which serves to isolate the input to the reverberationchannel from the direct signal channel. The output of isolationamplifier 22 is connected through a variable attenuator 24 to the inputof a conventional recording amplifier 26, the output of which isconnected to a recording head 28. Recording head 28 is suitablypositioned along the path of travel of endless recordable material 30,which for example, may be magnetic tape as shown in FIG. 1, or arotatable drum with magnetic material on its outer perephery. However,any type of recording apparatus having a continuous recordingcharacteristic and on which an original signal may be impressed at onepoint and recovered at one or more later points, may be employed. Forexample, an electrostatic recording system or ap-hotoelectric-phosphorescent system may be used, if desired.

Endless magnetic tape 30 is supported by idler rollers 32 and is adaptedto be driven by a constant speed motor 34 in the direction indicated bythe arrows. Also mounted adjacent the path of travel of tape 30 are aplurality of playback heads 36, 38, and an erase head 42 which may belocated at any selected position along the path of tape travel betweenplayback head 40 and record head 28. Erase head 42 is adapted tocontinuously erase any signal recorded on tape 30 immediately prior tothe recording of a new signal by recording head 28 and is accordinglyconnected to a convention biaserase oscillator 44. This oscillator mayoperate at a frequency of 25 kc. or above, and also furnishes a biassignal to record head 28.

Since by definition, reverberation comprises multiple repetitions ofsound, physically caused by multiple reflections which the earinterprets as a prolonged decay of the sound after the source hasceased, then an artificial system must furnish a number of repetitionsof the original sound; and the period of the repetitions and decrementthereof must be carefully selected. Thus, the intervals betweenrepetitions should be substantially constant while the periods betweenrepetitions should be sufliciently long so that the unnatural effect ofa small room with excessive reverberation is not produced. On the otherhand, if the period between repetitions is too long, then pulse-likesounds such as speech, or music with staccato notes, tend to producediscrete repetition pulses which are heardas discrete echoes.

. However, it has been found by experiment that repetitions in the rangeof 100 to 150 milliseconds are a satisfactory compromise. By adjustingthe spacings between record head 28, and playback heads 36, 38 and 40respectively, in cooperation with a selected speed of tape 30, thedesired repetition period may be achieved. For example, if the speed oftape 30 were 7 /2 inches per second, then for a selected reverberationperiod of milliseconds the spacing between heads 28, 36, 38, 40respectively, would be approximately .82 of an inch.

Playback head 36 is connected to the input of playback amplifier 46, theoutput of which, in turn, is connected through resistor 48 to the inputof control amplifier 50 to form playback channel #1. The output ofcontrol amplifier 50 is connected through resistor 52 to the output ofisolation amplifier 18. Likewise, playback head 38 is connected to theinput of a playback amplifier 54, the output of which, in turn, isserially connected with vari able attenuator 56 and resistor 58 to theinput of control amplifier 5t) to form playback channel #2. Playbackhead 40 is, in a similar manner, connected to the input of playbackamplifier 60 the output of which, in turn, is connected through avariable attenuator 62 to the input of a control amplifier 64 to formplayback channel #3. The output of control amplifier 64 is connectedthrough resistor 66 to the output of isolation amplifier 18. Alsoconnected in parallel with the input to control amplifier 64 is theinput of a feedback amplifier 68, the output of which in turn isconnected through a resistor 70 to the output of isolation amplifier 22.

In operation the signal recorded on tape 30 by record head 28 isdetected after a selected time interval has occurred, such intervalcorresponding to the chosen repetition period of the reverberationsystem, amplified by amplifier 46, control amplifier 50, and is mixedwith the direct signal appearing at the output of isolation amplifier18. In a like manner, after an additional time interval has occurred,the signal is detected by playback head 38, amplified by amplifiers 54and 50, and is also mixed with the direct signal appearing at the outputof isolation amplifier 18.

Similarly, the signal detected by playback head 40 is amplified byamplifiers 60 and 64, and mixed with the direct signal appearing at theoutput of isolation amplifier 18. In addition, the signal appearing atthe input of control amplifier 64 is transmitted to feedback amplifier68 where it is amplified and mixed with the direct signal appearing atthe output of isolation amplifier 22. The feedback signal is thusre-recorded to establish cyclic repetitions of the original signal witheach repetition occurring at successively .lower amplitudes.

An important feature of the invention is the arrangement of theattenuating devices in the record, playback and feedback channels sothat (1) the magnitude of each repetition signal is less than themagnitude of the next preceding repetition signal by a substantiallyuniform amount regardless of the number of complete cycles ofrepetition, and (2) the uniformity of difference between signal levelsis maintained, and only the slope of the reverberation curve is changed,when the variable attenuation controls 24, 56, 62 are manipulated tosecure reverberation times less than maximum. These objectives aregraphically shown in FIGS. 2a; 2b and 3.

In FIG. 2a, the effect of attempting to control reverberation time bymerely attenuating the amount of reverberated signal mixed with thedirect signal, a well known prior art technique, is graphicallyillustrated by curves 202, 204. Assuming that the direct signal is at azero db reference level, that three playback heads are provided, andthat the attenuation between each playback head is fixed at 1 db, thenfor maximum reverberation time only, the reverberation curve 202 will bea substantially smooth, straight line having a slope of 1 db per .11second reverberation time. This is only true for maxim-um reverberationtime.

If the output of the reverberation channel is merely reduced in anattempt to reduce the reverberation time, then an attenuation curve suchas 204 results. An excessively large and unnatural drop in output leveloccurs immediately after the direct signal ceases, for example, theillustrated 8 db drop, and then the slope of the repetitive signal curveremains the same as the curve 202 obtained for maximum reverberationtime. This sudden drop in volume without change in attenuation slope iswholly unnatural and unobtainable under natural reverberationconditions.

Another prior art system is shown .in FIG. 2b wherein there isgraphically shown an attempt to control reverberation time by increasingfeedback attenuation. Such attempts produce distinctly audible drops insignal level for each cycle of repetition. Examples of curves obtainedby two different adjustments of controls in such a system are shown incurves 206, 208. It will be noted that while both curves 206, 208 have arelatively smooth initial attenuation from the level of the directsignal for the first cycle of repetition signals provided by the threeheads, yet upon the commencement of the second cycle at point 210, thereis an audible drop in signal level provided by the increased attenuationof the feedback control. Then the sec-0nd repetition cycle commenceswith the same attenuation slope as for the first cycle. In the case ofcurve 208 shown in dotted outline, the audible drop at point 210 is evengreater, since, in the example illustrated, the feedback signal isattenuated 4 db. Controlling reverberation time by this system producesan unnatural reverberation signal having audible discontinuities due tothe nonuniform attenuation curve.

In FIG. 3 is shown a plurality of curves 212, 214, 216 respectively,obtained with the reverberation system of the present invention.Regardless of the length of the reverberation time, the attenuationcurves are smooth and free of bumps, since only the slope is change-d, anatural occurrence and analogous to physical reverberation changes.These curves are obtainable, as pointed out above, by changing uniformlythe attenuation between the several playback heads and feedback head sothat the difference therebetween is always the same. In curve 212, theattenuation bet-ween each reverberation time signal, spaced .11 second,is 1 db. In curve 214, the attenuation between signals is 2 db, thusproviding a different slope than curve 212, Whereas in curve 216 theattenuation shown in 4 db, again providing a shorter reverberation timethan either curve 212 or 214.

Referring again to FIG. 1 the novel attentuation arrangements of theseveral signal channels in the reverberation channel to achieve theresults shown by the curves of FIG. 3, will now be explained in detail.

It will be noted that attenuators 24, 56 and 62 are variable, and may beconventional potentiometers with selected tapers. When these controlsare set in an off position and thus do not attenuate the signals passingthrough the associated circuits, the reverberation channel 12 will thenbe adjusted for maximum reverberation. The slope of the maximumreverberation signal attenuation curve will then be determined by thepermanent attenuating devices still operative in the system whencontrols 24, 56, 62 are off. These permanent attenuating devices are alladjusted so that each recorded signal differs in magnitude from anadjacent, time-spaced signal by the same amount.

For example, assume that each signal repetition ditfers in magnitudefrom the next preceding repetition signal by 1 db. Then the gain ofamplifiers 46, 50 and the proportionin-g of voltage dividers 48, 58 and52, 66 are such that the signal picked up by the playback head 36, 110milliseconds after it is recorded by record head 28, appears at theoutput of isolation amplifier 18, 1 db lower than the direct signaloutput of this amplifier. Likewise, the signal picked up by playbackhead 38, 220 milliseconds after it has been recorded by record head 28,is adjusted in magnitude by adjustment of the gain of playback amplifier54, the proportioning of voltage divider 48, 58, the gain of controlamplifier 50 and proportioning of voltage divider 52-66, so that thesignal appearing at the output of isolation amplifier 18 is 1 db lessthan the signal provided by playback head 36 and 2 db less than that 6of the direct signal applied to input terminal 11. It is assumed thatattenuator control 56 is in the straight through or off position.

Three hundred and thirty milliseconds after it has been recorded byrecord head 28, the signal picked up by playback head 40, is adjusted inmagnitude by adjusting the gain of playback amplifier 60, controlamplifier 64, and the proportioning of voltage divider resistors 52, 66so that the signal appearing at the output of isolation amplifier 18 is1 db less than the signal originating from playback head 38 or 3 db lessthan the direct signal applied to input terminals 11 and appearing atthe output of isolation amplifier 18. In this instance, variableattenuator control 62 is in the straight through or off position.

The signal from playback head 40 is also amplified by feedback amplifier68 and transmitted through resistor 70 to join the direct signal at theoutput of isolation amplifier 22, but the gain of feedback amplifier 68and the value of resistor 70 are so designed that the feedback signalappearing at the output of isolation amplifier 22 is 3 db less than thedirect signal at the same point, obtained from the input of amplifier22. The signal from feedback amplifier 68 is accordingly amplified byrecord amplifier 26 and re-recorded by record head 28 to thus start asecond cycle of repetitive signals.

As in the case for the first cycle, the re-recorded feedback signal ispicked up by the playback head 36 amplified by playback channel #1 andagain appears at the output of isolation amplifier 18. As channel #1provides 1 db attenuation with respect to the magnitude of the directsignal, and as the feedback signal has been recorded at a level 3 dbless than the direct signal, the resulting magnitude of the re-recordedsignal provided by playback channel #1 to the input of amplifier 20 istherefore minus 4 db with respect to the direct signal arriving at thesame point. It will be seen that in a like manner, the repetitionprovided by playback channel #2 on the sec-0nd cycle will be attenuated5 db, and likewise the signal provided by playback channel #3 will beattenuated 6 db.

Accordingly, every repetition of every cycle differs in magnitude fromadjacent time spaced signals by the same amount. However, asreverberation time is definition is the time required for a repetitionsignal to be 60 db less than the direct signal from which it originated,then in the example just given, as each repetition is 1 db less than theimmediate preceding repetition, 60 repetitions are required to fulfillthe definition. As the time delays between repetitions are arbitrarilychosen at .11 second, then for this example the maximum reverberationtime is 60 .11 or 6.6 seconds.

The operation of the system to maintain uniform the attenuationdifferences between adjacent signal repetitions for reverberation timesshorter than the maximum of the system will now be described.

The reverberation time is easily shortened from the maximumreverberation time set for the system to 0 by simply varying attenuators24, 56 and 62, which preferably are ganged together for tandemoperation. These attenuators may be a plurality of series connectedresistors with switchable output taps between each resistor, orpotentiometers with selected tapers if a continuous adjustment frommaximum to 0 is desired.

Although the maximum resistance values of the attenuator controls may bedifferent due to different impedance requirements of the circuits inwhich they are connected, they have definite relationships to each otherwith respect .to the amount of attenuation for given r0- tationalpositions, or switch positions. As will be explained in detail below,for a system having three playback channels, controls 24 and 56 have thesame rate of attenuation in db per degree of shaft rotation or for eachswitch position, whereas control 62 has twice the rate of attenuation ofcontrols 24 and 56. In the case of four playback channels, theattenuation control therein corresponding to control 62 would have arate of attenuation triple the attenuating rate for control 24 and 56.Thus, for example, in a three playback channel system, if rotation ofthe ganged attenuators 24, 56, 62 introduces an attenuation of db intothe direct channel and playback channel #2 respectively, then attenuatorcontrol 62 will have an attenuation of db.

To better understand this operation, assume that a reverberation time of3.3 seconds is desired, which in the case of three playback channelsrequires that each signal repetition be 2 db less than the immediatelypreceding signal. To fulfill the reverberation definition, repetitionsare required at .11 second time spacing therebetween to provide areverberation time of 3.3 seconds. It will be remembered that thepermanent attenuation with respect to the magnitude of the direct signalfor playback channels #1, #2 and #3 as 1 db, '2 db, -3 db respectively.Of course, any other absolute value of attenuation per channel may beselected as desired, pro- 8 level provided by each playback'head being 2db less than the immediately preceding signal. The relative outputlevels in db attenuation provided by the controls 24, 56, 62 and the dbdecrement between repetitions is summarized in the following table forseveral selected Summarized below are the attenuations obtainable if asystem having 5 playback heads is employed and a reverberation time of3.3 seconds is desired.

vided the uniformity of difference in attenuation between adjacentchannels is maintained.

Then, if the signal provided by playback channel #1 is to appear at theoutput of isolation amplifier 18 with a magnitude of 2 db less than thedirect signal appearing at this point, control 24 must introduce 1 dbloss into the record channel. The 1 db so provided, in addition to the 1db permanent attenuation provided by playback channel #1 will give thedesired 2 db attenuation to the output signal of playback channel #1. Asthe output signal from playback channel #2 must be 2 db less than theoutput signal of channel #1, or 4 db less than the direct signal,control 56 must introduce 1 db to playback channel #2. This is due tothe fact that the signal level in playback channel #2 is reduced 1 dbdue to the record level of the signal provided by the record channel and2 db by the permanent attenuation of playback channel 2 so that control56 must provide 1 db for the required 4 db total attenuation.

In a like manner the signal level of playback channel #3 must be reduced6 db with respect to the direct signal appearing at the output ofisolation amplifier 18, or 2 db less than the signal provided byplayback channel #2. As the record signal is reduced 1 db by control 24,and the permanent attenuation of playback channel #3 is 3 db, thencontrol 62 must contribute 2 db to fulfill the -6 db requirement. Itwill thus be seen that the attenuation value of control 62 is twice thatof controls 24 and 56.

As the attenuation of the feedback signal is the same as that of thesignal level of playback channel #3, as mentioned above, the signalfurnished by feedback amplifier 68 to be rerecorded is therefore 6 dbless than the direct signal at the output of amplifier 22. This -6 dbsignal is additionally attenuated by the 1 db provided by control 24,recorded by record head 28, picked up by playback head 36, and fedthrough playback channel #1 where it is further attenuated 1 db by thepermanent attenuation of this channel and appears at the output ofisolation amplifier 18 at a level 8 db less than the direct signal.

A second repetition cycle has thus commenced with the uniformity ofdifference in magnitude between adjacent repetition signals still beingpreserved. Accordingly, curve 214 of FIG. 2c is produced with the signalBased on the foregoing, the attenuation requirements for the severalchannels forming the overall reverberation channel 12 may now besummarized and expressed in general mathematical notations as follows:

(1) The variable attenuation control of any channel,

VA (db)'= (N-1)xA where N=the playback channel number, and A =thevariable attenuation of the first playback channel (with respect to thedirect signal). (2) The permanent attenuation of any selected playbackchannel PA NXPA (3) The selected total attenuation for playback channel#1, with respect to that of the direct channel, determines the minimumpermanent attenuation for all playback channels.

Referring now to FIG. 4, a schematic circuit diagram of a reverberationsystem in accordance with the present invention having 3 playbackchannels, is shown. Where units are identical, the reference numerals ofFIG. 1 are used. A signal from a source of audio signals such as anelectronic musical instrument is fed to input terminals 11 throughpotentiometer 302 by means of movable arm 304 to the input grid of anelectronic valve 306. Potentiometer 302 is connected in series with aresistor 308 to ground, and the control electrode of valve 306 isconnected to the junction point thereof. Valve 306 is included in thecircuitry of isolation amplifier 22, which serves to separateelectrically, reverberation channel 12 from direct channel 10.

The control electrode of valve 310 forming part of isolation amplifier18, is also connected to the control electrode of valve 306 to enablethe input signal appearing across resistor 308 to be fed to isolationamplifier 18 of direct channel 10. The direct signal is amplified byvalve 310 and fed through capacitor 312 to an output level adjustingpotentiometer 314 which has one terminal connected in series with aresist-or 316 and ground. The movable arm 317 of potentiometer 314transmits the signal developed t'hereacross through capacitor 318 to thecontrol electrode of a valve 320 included in output amplifier 20, whichis connected in a cathode follower configuration. The output ofamplifier 20 is fed through a capacitor 322 to a pair of outputterminals 16.

An important feature of the present invention is the arrangement of theinput and output gain controls in order to minimize the system noiseWherever possible. It will be noted that the movable arms ofpotentiometers 304, 317 of potentiometers 302, 314 respectively, areganged to each other. These potentiometers are preferably,s-ubstantially identical in taper although they need not be necessarilyidentical in absolute resistance value. However, the ratio of theresistance of resistor 308 to the resistance of control potentiometer302 is preferably substantially identical to the ratio of the resistanceof resistor 316 to its associated potentiometer 314. For example,potentiometers 302, 314 may be one hundred thousand ohms wherearesistors 308, 316 may be ten thousands ohms. Thus, the input and outputvoltage may be adjusted over a range of approximately 11-1.

In operation, it is preferable to adjust control 302, 314 to as low apoint with reference to ground as postible, which accordingly willprovide, due to voltage divider action, maximum expected signal toreverberation channel 12. When it is desired to apply maximum inputsignal to terminals 11, movable arms 304, 317 are at maximum resistancepositions of potentiometers 302, 314, with respect to ground. Under thiscondition, the input signal will be attenuated to the level required forthe maximum recording signal in reverberation channel 12, whereas theoutput control 314 is set to provide maximum signal output to terminal16. The gains of amplifiers 18, 20 are such that under this maximumoutput condition, the output signal is at the same level as the inputsignal for direct channel 10.

If only a minimum signal is available for application to input terminals11, then movable arms 304, 317 are set to a minimum resistance positionfor potentiometers 302, 314, with respect to ground. Because of thevoltage divider action of these controls, the full input voltage isapplied to both the reverberation and directs channels. The maximumvoltage is then still delivered to reverberation channel 12. However,since the gain of this system remains unchanged, the location of movablearm 317 at low resistance point of its ossociated control thereby causesthe direct signal to be attenuated so that the signal output fromchannel appearing across output terminal 16 again remains the same asthe input signal level. However, under these conditions, any noise orhum which may have originated in the system is also attenuated.

Thus, it will be seen that in accordance with this feature of theinvention, for high level input signals, no attenuation of internallygenerated noise is provided, since none is needed under high signallevel conditions. On the other hand, when the input signal is at a lowlevel, maximum noise attenuation is provided. Thus, the signalto-noiseratio of the system remains substantially constant over a large range ofavailable input signals, rather than decreasing with signals of smalleramplitude.

Continuing with the description of the circuitry of reverberationchannel 12, the signal developed across resistor 308 and applied to theinput electrode of valve 306 is amplified thereby and appears acrossseries connected resistors 324, 326 which are connected to a source ofpositive voltage 327. Voltage divider action is provided by resistors324, 326 so that the signal, at reduced output level, is transmittedthrough capacitor 328 to one terminal 329 of control 24.

Control 24 comprises a plurality of serially connected resistors 330,332, 334, 336, 338 with the last terminal of the series string connectedto ground. The junction points between the serially connected resistors330-338, are connected to taps which are selectively engaged by movablearm 340. Also connected to terminal 329 is the output signal from valve394 of feedback amplifier 68, which is developed across resistor 342 andfed through capacitor 344 and resistor 346 to "terminal 329. Thecombined gains of isolation amplifier 22 and feedback amplifier 68 arepreferably adjusted so that the signal appearing at terminal 329 has anamplitude operative to drive recording amplifier 26 to the maximumoutput required by the record head when movable arm 340 engages tap 329in the zero attenuation position of control 24.

The reverberation signal is next amplified by record amplifier 26 andfed through capacitor 343 and transformer 345 to record head 28. Valve.347 functions as a conventional bias and erase oscillator operating ata frequency suitable for the heads in use. Thus, the erase signal is fedto erase head 42 through capacitors 348, 350 respectively, and biassignal for record head 28 is fed thereto through transformer 345. As thecircuit arrangements for record amplifier 26 and bias and eraseoscillator 44 are conventional and well known in the art, no furtherdescription thereof is necessary.

The recorded signal from the tape is picked up by playback heads 36, 38,40, each connected to associated amplifiers 46, 54 and 60 respectively,which are conventional two-stage voltage amplifiers. The output ofamplifier 46 is fed through capacitor 352 and isolation resistor 48 tothe input electrode of valve 356. One output terminal of amplifier 54 isconnected through a capacitor 358 to a terminal 372 of attenuatorcontrol 56. This control comprises a plurality of serially connectedresistors 362, 364, 366, 368, 370 connected between terminal 372 andground. The junction points between resistors 362-370 are connected totaps which are engaged by wiper arm 374 which in turn is seriallyconnected through an isolation resistor 58 to the input electrode ofvalve 356. The output signal from amplifier 54 is thus developed acrossattenuator resistors 362-370, and by voltage divider action, is appliedto the input electrode of valve 356 through a circuit path comprising aselected tap of attenuator 56, wiper arm 374, and resistor 58.

The signal from playback amplifier 60 is fed through capacitor 376 toterminal 378 of attenuator 62, which comprises a plurality of seriallyconnected resistors 380, 382, 384, 386, 388 connected between terminal378 and ground. The junction point between each resistor is con nectedto an output tap which is engaged by a wiping arm 390 which is connectedto the input electrode of valve 392 of control amplifier 64.

The input electrode of valve 394 of feedback amplifier 68 is alsoconnected to the input electrode of valve 392. The signal receivedtherefrom is amplified by valve 394 and fed to the input of recordamplifier 26 in the manner described hereinbefore. The permanentattenuation of the signal is, of course, set to the point describedhereinbefore, by adjustment of the impedances of amplifier 68, such asresistors 341, 346 and capacitor 344.

The outputs of control amplifiers 50, 64 are respectively fed throughcapacitors 354, 360 to resistors 52, 66 which have a common junctionpoint connected to the higher resistance terminal of output levelpotentiometer control 314 Where the signal output from reverberationchannel 12 is mixed with the direct signal input from isolationamplifier 18 of direct channel 10.

Resistor 58, associated with playback amplifier 54, is so adjusted inmagnitude with respect to resistor 48 associated with playback amplifier46 that, when all of the shunt impedances of the two amplifiers areconsidered, the signal appearing at the control electrode of valve 356derived from amplifier 54 is 1 db less than that derived from amplifier46, which is the permanent attenuation mentioned above and provides 1 dbattenuation between playback channels if desired. Likewise, the valuesof output resistors 52, 66 are so adjusted that the voltage divideraction provided thereby is operative to cause a permanent attenuation of3 db with respect to the direct or recorded signal in order that theadditional 1 db attenuation between playback channel 2 and playbackchannel 3 is achieved. As there are three playback channels, the valuesof resistors 378-388 of attenuator control 62 should be selected so thatfor each setting of the ganged attenuator controls 24,56, 62, theattenuation in decibels provided by control 62 is twice thatof the othercontrols.

While step attenuators having six positions are shown for purposes ofillustration, it will be understood that attenuators having more or lesspositions may be used, if desired, or ganged potentiometers withsuitably selected tapers and having an infinite number of positions mayalso be used.

Although a reverberation system utilizing three playback channels hasbeen described for purposes of illustration, it will be understood thatby following the principles of the present invention, additionalplayback channels including heads and amplifiers, may be added ifdesired, or even a two-channel playback system may be employed.

While the present invention has been disclosed by means of specificillustrative embodiments thereof, it would be obvious to those skilledin the art that various changes and modifications in the means ofoperation described or in the apparatus, may be made Without departingfrom the spirit of the invention as defined in the appended claims. I

What is claimed is:

1. In a synthetic reverberation system having input terminals coupled toa' direct-signal channel and to a reverberation signal channel andhaving means for combining the outputs of both channels to provideoutput for the system, and the reverberation signal channel includingdelay means combined with plural means for generating from an inputsignal successive series of time-spaced signals of decreasing amplitude,and further including feedback means coupled to receive the last of thegenerated signals in each series andreintnoduce it into said delay meansto generate a new series, improved variable attenuating means forcontrolling the amplitudes of said spaced signals comprising:

(a) separate variable attenuation means controlling the relative outputamplitudes of said signals from i the plural generating means to saidcombining means;

(b) means at the input of the reverberation signal channel for addingthe feedback and the input signal together;

(c) variable attenuation means coupling the adding means to the delaymeans;

(d) means for ganging together all said attenuation means forsimultaneous adjustment tomultiple selectible positions; and

(e) the attenuation means coupled to the adding means having anattenuation characteristic tapered to cooperate with matched tapers ofthe attenuation means which .are coupled to the generating means formaking the amplitudes of all delayed signals in successive series alwaysdecrease along a smooth curve with respect to time for all saidpositions.

2. In a system as set forth in claim 1, said variable attenuation meansbeing adjustable to equal zero, and fixed attenuation means in seriestherewith and selected to provide a predetermined minimum attenuationbetween successive signals, and the variable attenuation associated witheach of said generating means after the first thereof beingsubstantially equal to (N1)A Where:

N=the number in succession of the generating means other than the firstone, and

A =the variable attenuation of the first of said generating means beyondsaid predetermined attenuation.

3. In a system as set forth in claim 2, the fixed attenuation of any ofsaid generating means other than the first thereof being substantiallyequal to N PA where:

PA =the fixed attenuation of the first of said generating means.

4. In a system as set forth in claim 1, the attenuation means for saidadding means and for the first of said generating means havingsubstantially identical tapers, and the other attenuation means havingtapers equal to (N-l) multiplied by the taper of the adding meansattenuation, where N=the number in succession of the generating meansother than the first one.

5. In a system as set forth in claim 4, the variable attenuation meansof the adding means being common to both the direct input to thereverberation signal channel and the feedback signal input to thereverberation signal channel and the feedback signal input to thereverberation channel.

References Cited by the Examiner UNITED STATES PATENTS 2,493,638 1/1950Olson- 1791.6 2,674,660 4/1954 Ambrose 179-1002 2,712,040 6/1955 HeytoWet al 330144 2,757,245 7/ 1956 Pihl 330-444 3,057,969 10/1962 Stolle etal l79100.2 3,062,923 11/1962 Stolle 179-1002 3,069,956 12/1962 Bode84-124 3,073,900 1/1963 Victoreen 330-155 3,095,482 6/ 1963 Whitefordl79100.2 3,110,771 11/1963 Logan etal 179l.6

I BERNARD KONICK, Primary Examiner.

1. IN A SYNTHETIC REVERBERATION SYSTEM HAVING INPUT TERMINALS COUPLED TOA DIRECT-SIGNAL CHANNEL AND TO A REVERBERATION SIGNALS CHANNEL ANDHAVING MEANS FOR COMBINING THE OUTPUTS OF BOTH CHANNELS TO PROVIDEOUTPUT FOR THE SYSTEM, AND THE REVERBERATION SIGNAL CHANNEL INCLUDINGDELAY MEANS COMBINED WITH PLURAL MEANS FOR GENERATING FROM AN INPUTSIGNAL SUCCESSIVE SERIES OF TIME-SPACED SIGNALS OF DECREASING AMPLITUDE,AND FURTHER INCLUDING FEEDBACK MEANS COUPLED TO RECEIVE THE LAST OF THEGENERATED SIGNALS IN EACH SERIES AND REINTRODUCE IT INTO SAID DELAYMEANS TO GENERATE A NEW SERIES, IMPROVED VARIABLE ATTENUATING MEANS FORCONTROLLING THE AMPLITUDES OF SAID SPACED SIGNALS COMPRISING: (A)SEPARATE VARIABLE ATTENUATION MEANS CONTROLLING THE RELATIVE OUTPUTAMPLITUDES OF SAID SIGNALS FROM THE PLURAL GENERATING MEANS TO SAIDCOMBINING MEANS; (B) MEANS AT THE INPUT OF THE REVERBERATION SIGNALCHANNEL FOR ADDING THE FEEDBACK AND THE INPUT SIGNAL TOGETHER; (C)VARIABLE ATTENUATION MEANS COUPLING THE ADDING MEANS TO THE DELAY MEANS;(D) MEANS FOR GANGING TOGETHER ALL SAID ATTENUATION MEANS FORSIMULTANEOUS ADJUSTMENT TO MULTIPLE SELECTIBLE POSITIONS; AND (E) THEATTENUATION MEANS COUPLED TO THE ADDING MEANS HAVING AN ATTENUATIONCHARACTERISTIC TAPERED TO COOPERATE WITH MATCHED TAPERS OF THEATTENUATION MEANS WHICH ARE COUPLED TO THE GENERATING MEANS FOR MAKINGTHE AMPLITUDES OF ALL DELAYED SIGNALS IN SUCCESSIVE SERIES ALWAYSDECREASE ALONG A SMOOTH CURVE WITH RESPECT TO TIME FOR ALL SAIDPOSITIONS.